1. Field of the Invention
This invention relates generally to feedback elements for servo systems, and particularly to potentiometer type feedback elements. More particularly, this invention relates to a digital continuous potentiometer servo feedback element for providing smooth operation of the servo for the full displacement of the potentiometer arm.
2. Description of the Prior Art
Servo systems used in aircraft instrumentation are of the digital continuous closed loop type including a feedback element in the servo loop. Since synchros have the characteristic of 360.degree. angular displacement and provide a continuous output without deadband, prior to the invention of U.S. Pat. No. 3,662,246 they had been commonly used as the feedback element. This was because conventional potentiometers have limited angular displacement (about 350.degree.) and hence are not suitable for the purposes described.
The potentiometer of U.S. Pat. No. 3,662,246 overcomes this difficulty and provides several additional advantages. Included among these are that simple digital to analog converters rather than more complicated digital to synchro converters may be used in the system and descrete most significant (MSB) inputs from digital signals may be applied. Further, an all d.c. feedback loop is thus provided.
While this type of potentiometer feedback element performs reasonably well for the purposes intended, one significant problem remains. Consider that the potentiometer servo feedback element of U.S. Pat. No. 3,662,246 has its excitation switched through a rotary switch every 180.degree. to generate two output ramps per revolution of the potentiometer arm. However, the potentiometer excitation switch point, which is determined by the rotary switch, cannot always be accurately aligned to the potentiometer terminations. This causes instrument display errors. In addition, during slew conditions the instantaneous error into the servo changes abruptly at the switch point. Thus, the servo changes speed every time it switches, i.e., at the 0.degree. and 180.degree. points. The device of the present invention is an improvement over the prior art potentiometer feedback element described in U.S. Pat. No. 3,662,246 in that a constant excitation is provided which eliminates the aforenoted switching and the errors associated therewith.